These breathable foam stoppers can stand up to boiling temperatures, making them great to use when preparing a yeast starter. The foam stoppers will act as a filter for any airborne bacteria or wild yeast that might enter the flask. This is important because a lot of air is sucked into the flask while it is cooling. You can also leave the stoppers on the flask instead of a fermentation lock. Some people use foam stoppers to replace fermentation locks in their carboys, too.
The small foam stoppers are 35-45mm wide and fit our 500 or 1000 ml flasks, or a glass carboy.